292 HISTOLOGY OF THE MORBID PROCESSES. 



plasia from fibrous tissue lacks the elaborate system of canaliculi 

 that is found in normally developed osseous tissue, although in its 

 essential features it is virtually bone, the intercellular substances 

 being impregnated with calcareous matter and yielding gelatin on 

 boiling. 



Epithelial tissues may also be the seat of metaplasia. Under the 

 influence of moderate but repeated damage, columnar epithelium 

 may become modified into a stratified variety. In such cases the 

 cause may, presumably, be traced to a change of conditions, which 

 calls for an unusual exercise of the protective function of the epi- 

 thelium. The uterine cavity and the respiratory tract are the most 

 common situations in which this transformation of epithelium is 

 met with. A similar conversion of transitional epithelium into true 

 stratified epithelium is occasionally met with in the bladder and 

 renal pelvis, as the result of a calculus not causing sufficient damage 

 to induce an active inflammation. 



Metaplasia appears to result from a change in the functional 

 activities of the cells, which lose their accustomed form of special- 

 ization and acquire new ones of closely related character. 



